Abstract
Studies of pre-Jurassic section (PJS) of West-Siberian plate were in progress since the end of 1940s, and were closely related to petroleum exploration. Up to the mid-1950s views on tectonic and geology of the basement were based on general geological understanding of mountain structures surrounding the West-Siberian plate. Deep drilling in West Siberia was initially based on the concept of oil productivity of Palaeozoic formations by analogy with the Volga-Urals basin. Oil discoveries in Mesozoic and Cenozoic formations within the sedimentary cover in 1960s have not stopped studies of older formations - many wells are still being drilled entering a sedimentary cover bottom. However, only approximately 50 reservoirs were discovered within the PJS during the past 70 years (as compared with approximately three thousand reservoirs in Jurassic and Cretaceous formations); most of them are associated with the weathered crust. Reservoir model are generally represented as non-stratiform traps limited to highs of various types: tectonical, erosional, etc. [Kleshchev, Shein, 2010]. Oil genesis in such reservoirs is widely discussed in terms of abiotic oil hypothesis.
Wide-azimuth 3D seismic survey in Sredne-Nazymsky oil field coupled with the modern processing procedures, including depth migration and interpretation support of processing using Well Driven Seismic methodology, allowed obtaining clear image of stratified section in the PJS. The substantial for PJS of West Siberia and oil inflow proved by testing from one of the wells triggered a project on detailed investigations of pre-Jurassic section. During the project, different hypotheses of geological structure and productivity factors for the pre-Jurassic section were examined. On the basis of classic petroleum theory, the exploration criteria were substantiated for the sweet spots within pre-Jurassic formations of the area of interest. The detailed and accurate continuous seismic events interpretation provided an input for PJS structural and tectonic model. Using analogy with the neighbouring areas, it was shown that the sediments are formed by igneous-sedimentary formations successively truncated by the pre-Jurassic erosion surface.
The assumption was made that the massive oil reservoir exists within the PJS; shaly Jurassic rocks serve as a seal for PJS deposits and were also most likely the source rocks. The Sredne-Nazymsky field is located at the dome of the same name on the western slope of the Frolovsky depression (Figure 1). Apparently, this depression serves as a petroleum kitchen - pre-Jurassic rocks of the Sredne-Nazymsky field was charged by laterally ascending HC migration from the deeply buried Jurassic source rocks in the Frolovsky depression. Acoustic inversion of seismic data helped to delineate elastically contrast interbedded layers. It was shown that layers with lower acoustic impedance correspond to reservoir rocks. An assumption was also made about the important contribution of natural fractures to productivity of pre-Jurassic rocks, and zones of their development were predicted. Based on the substantiated exploration criteria, recommendations were given on exploration wells positioning.
Subsequent drilling of wells penetrating pre-Jurassic rocks within exceeding 200 m interval fully proved the structural and tectonic model and reservoir prediction made by seismic interpretation. Commercial oil inflows and formation water were obtained from the wells drilled. Same depth of water shows in two wells situated far from each other is indirectly indicative of a unique oil-water contact.
